The present invention relates to a method and apparatus for transmitting signals in an instrument such as a length or angle measuring instrument of the type comprising a first signal line on which a periodic signal with first and second signal states is transmitted, a second signal line, and means for generating first and second nonperiodic signals.
German DE-OS 30 39 483 discloses a known incremental measuring system which includes a scale that defines a graduation and a plurality of identical reference marks absolutely positioned with respect to the graduation. Respective code marks are allocated to each of the reference marks for identification purposes. The graduation, the reference marks, and the associated code marks of the measuring scale are scanned by means of associated scanning fields of a scanning unit included in the measuring instrument. The scanning unit generates two periodic scanning signals as the scanning unit moves along the graduation of the scale. In order to allow discrimination of the measuring direction, these two periodic scanning signals are phase shifted with respect to one another through 90.degree. (a quarter of the graduation period of the graduation). The two scanning signals are applied via two respective scanning signal lines to an evaluating arrangement for the determination of the relative position of the scanning unit with respect to the scale. In addition, the scanning unit generates a reference signal during scanning of the reference marks, and these reference signals are applied via a reference signal line to the evaluating arrangement. Similarly, the scanning unit generates code signals in response to scanning of the code marks, and the code signals are applied via a code signal line to the evaluating arrangement. In such a measuring arrangement, the electrical reference signals generated in response to the reference marks can be used in various ways in the evaluating arrangement. For example, they can be used to reproduce the zero position of a counter including in the evaluating arrangement, or for loading a predetermined position value into the counter at the beginning of the measurement operation, or for control or checking of interference pulses. In addition, such reference signals can be used to control external arrangements coupled on the outlet side of the evaluating arrangement.